Bare metal laser shock peening

ABSTRACT

A method of making a laser shock peened article includes bare laser shock peening a bare metallic surface of a substrate of the article without using an ablative coating and forming a pre-stressed region having deep compressive residual stresses extending into the article and a recast layer above the pre-stressed region. Removing just the recast layer with an abrasive vibratory process with an abrasive media. The vibratory process may include one or more of the following: abrasive tumbling including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle, mechanical peening including peening the article with small particles which either abrade and/or shatter a brittle surface of the recast layer, and abrasive polishing including loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article.

BACKGROUND OF THE INVENTION

This invention relates to laser shock peening and, more particularly, to methods for laser shock peening uncoated surfaces.

Laser shock peening (LSP) or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high and low power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477 entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957 entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of the surface. The portion of the surface may have an ablative coating or be bare meaning having no ablative coating. An explosive force is produced at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of the material surface or of a coating (such as tape or paint) on the surface which forms a plasma.

Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,736,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No. 5,591,009 entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,531,570 entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447 entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328 entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee.

High energy laser beams, from about 20 to about 50 Joules, or low energy laser beams, from about 3 to about 10 Joules, have been used and other levels are contemplated. See, for example, U.S. Pat. No. 5,674,329 (Mannava et al.) issued Oct. 7, 1997 (LSP process using high energy lasers) and U.S. Pat. No. 5,932,120 (Mannava et al.) issued Aug. 3, 1999 (LSP process using low energy lasers). Low energy laser beams can be produced using different laser materials such as neodymium doped yttrium aluminum garnet (Nd YAG), Nd:YLF, and others. Laser shock peening processes typically employ a curtain of water or other confinement liquid medium flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shockwaves into the bulk of the material of a component being LSP'D, to create the beneficial compressive residual stresses.

The LSP process generates deep compressive stresses in the article resulting in improved fatigue strength under foreign object damage (FOD) conditions. LSP improves material properties such as high cycle fatigue, low cycle fatigue, corrosion & erosion resistance. However, this process also generates residual tensile stresses which can cause the loss of fatigue strength and HCF and LCF capability. It is desirable to LSP articles such as rotating gas turbine engine components without using an ablative coating because of the extra cost and time associated with coating and recoating surfaces of the components that are laser shock peened. Laser shock peening of leading and/or trailing edges of fan, compressor, and turbine blade airfoils may require several coatings and recoatings of the surfaces that are laser shock peened. Devices used to coat the components are expensive and/or difficult to maintain in the noisy and dirty environment of the LSP process.

The benefit of coating surfaces during the LSP process include lower ignition thresholds and enhancing the magnitude of the LSP effect. If the coating is eliminated, then the surface of the component will typically be burned either from the laser radiation directly or the broadband radiation from the expanding blast wave (plasma) above the surface of the component, or both. Surface burning is of a concern because the component is cosmetically “scarred” and will appear different from similar components that are non-LSP'd or LSP'd with a surface coating. The scarring is melted and resolidified component material termed “recast” or “remelt”. Surface burning may also cause the component to exhibit a lower LSP effect as compared to an LSP process using ablative coating. The component can potentially fail (circumvent the LSP deep compressive stress) if cracks or defects can initiate in the burned surface layer and propagate around the LSP effected area.

Thus, it is highly desirable to provide a laser shock peening process on bare metal without the use of ablative coatings and avoid and counter the detrimental effects of surface burning typically associated with bare laser shock peening. It also highly desirable to provide such a process in an inexpensive manner. It is also desirable to remove the recast formed during bare surface laser shock peening.

BRIEF DESCRIPTION OF THE INVENTION

A method of making a laser shock peened article includes bare laser shock peening a bare metallic surface of a substrate of the article without using an ablative coating and forming a pre-stressed region. The pre-stressed region has deep compressive residual stresses extending into the article. The bare laser shock peening causes a recast layer to form above the pre-stressed region. A next step of the method is removing just the recast layer with an abrasive vibratory process with an abrasive media. The vibratory process may include one or more of the following: abrasive tumbling including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle, mechanical peening including peening the article with small particles which either abrade and/or shatter a brittle surface of the recast layer, and abrasive polishing process including loading the article into a bed of paste including abrasive particles, and oscillating the bed across and around the article.

One embodiment of the abrasive tumbling process includes packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle for a period of time in a range of several minutes to several hours and using vibratory frequencies in a range from a few Hz to several hundred Hz. One abrasive media includes stones characterized as having a size range from about 0.2 inches to 1.0 inches.

The method of making a laser shock peened article by bare laser shock peening without the use of ablative coatings and removing just the recast layer with an abrasive vibratory process is less expensive than using coatings and avoids and counters the detrimental effects of surface burning associated with bare laser shock peening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary aircraft gas turbine engine fan blade.

FIG. 2 is a cross-sectional view illustration of the fan blade in FIG. 1 after it has been bare laser shock peened.

FIG. 3 is a schematic perspective view illustration of a fan blade, similar to the blade in FIG. 1, mounted in a laser shock peening system illustrating an exemplary bare laser shock peening step of the method of the present invention.

FIG. 4 is a partial cross-sectional and a partial schematic view of the setup in FIG. 3.

FIG. 5 is an enlarged view illustration of a bare laser shock peened leading edge of the blade illustrated in FIG. 2.

FIG. 6 is an enlarged view illustration of a bare laser shock peened surface of the blade illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIGS. 1, 2, and 3, is a fan blade 8 having an airfoil 34 made of a Titanium alloy extending radially outward from a blade platform 36 to a blade tip 38. The fan blade 8 exemplifies an article 12 having a bare (uncoated) laser shock peening surface 54 of a hard metallic substrate 10 a portion of which is laser shock peened bare meaning without an ablative coating. The fan blade 8 includes a root section 40 extending radially inward from the platform 36 to a radially inward end 37 of the root section 40. At the radially inward end 37 of the root section 40 is a blade root 42 which is connected to the platform 36 by a blade shank 44. The airfoil 34 extends in the chordwise direction between a leading edge LE and a trailing edge TE of the airfoil. A chord C of the airfoil 34 is the line between the leading edge LE and trailing edge TE at each cross-section of the blade as illustrated in FIG. 2. A pressure side 46 of the airfoil 34 faces in the general direction of rotation as indicated by an arrow V and a suction side 48 is on the other side of the airfoil and a mean-line ML is generally disposed midway between the two faces in the chordwise direction.

The fan blade 8 has leading and trailing edge sections 50 and 70 that extend along the leading and trailing edges LE and TE, respectively, of the airfoil 34 from the blade platform 36 to the blade tip 38. The leading and trailing edge sections 50 and 70 includes first and second widths W1 and W2, respectively, such that the leading and trailing edge sections 50 and 70 encompass nicks 52 and tears that may occur along the leading and trailing edges of the airfoil 34. The airfoil 34 is subject to a significant tensile stress field due to centrifugal forces generated by the fan blade 8 rotating during engine operation. The airfoil 34 is also subject to vibrations generated during engine operation and the nicks 52 and tears operate as high cycle fatigue stress risers producing additional stress concentrations around them.

To counter fatigue failure of portions of the blade along possible crack lines that can develop and emanate from the nicks and tears, one or both of the pressure side 46 and the suction side 48 are bare laser shock peened forming laser shock peened surfaces 55 with a pre-stressed region 56 having deep compressive residual stresses imparted by laser shock peening (LSP) extending into the airfoil 34 from the laser shock peened surfaces 55 as seen in FIG. 2. The pre-stressed regions 56 are illustrated as being co-extensive with the leading and trailing edge sections 50 and 70 in the chordwise direction to the full extent of widths W1 and W2 and are deep enough into the airfoil 34 to coalesce for at least a part of the widths. The pre-stressed regions 56 are shown co-extensive with the leading edge section 50 in the radial direction along the leading edge LE but may be shorter.

Illustrated in FIG. 4 is a laser shock peening apparatus 1 including a laser beam apparatus having a generator 31 with an oscillator and a pre-amplifier and a beam splitter which feeds the pre-amplified laser beam into two beam optical transmission circuits each having a first and second amplifier 30 and 32, respectively, and optics 35 which include optical elements that transmit and focus the laser beam 2 on the bare or uncoated laser shock peening surfaces 54. The controller 24 may be used to modulate and fire the laser beam apparatus to fire the laser beam 2 on the laser shock peening surface 54 in a controlled manner.

A clear confining medium 68 to cover the laser shock peening surface 54 is provided by a curtain of clear fluid such as water 21 supplied by a water nozzle 20 at the end of a water supply tube 19. The curtain of water 21 is particular to the exemplary embodiment illustrated herein, however, other types of confining mediums may be used. The laser shock peening apparatus 1 illustrated herein includes a laser beam apparatus including a generator 31 having an oscillator and a pre-amplifier and a beam splitter which feeds the pre-amplified laser beam into two beam optical transmission circuits each having a first and second amplifier 30 and 32, respectively, and optics 35 which include optical elements that transmit and focus the laser beam 2 on the laser shock peening surface 54. The controller 24 may be used to modulate and fire the laser beam apparatus to fire the laser beam 2 on the bare laser shock peening surface 54 in a controlled manner.

The laser beam shock induced deep compressive residual stresses in the compressive pre-stressed regions 56 are generally about 50-150 KPSI (Kilo Pounds per Square Inch) extending from the bare laser shock peened surfaces 55 to a depth of about 20-50 mils into the compressive pre-stressed regions 56. The laser beam shock induced deep compressive residual stresses are produced by repetitively firing a high energy laser beam 2 that is defocused+a few mils with respect to the laser shock peening surface 54. The laser beam 2 typically has a peak power density on the order of magnitude of a gigawatt/cm² and is fired with a curtain of flowing water or other fluid that is flowed over the laser shock peening surface 54 or some other clear confining medium.

Bare metal of the metallic substrate 10 is ablated generating plasma which results in shock waves on the surface of the material. These shock waves are redirected towards the laser shock peening surface 54 by the clear liquid confining medium 68, illustrated herein as the curtain of water 21, or confining layer to generate travelling shock waves (pressure waves) in the material below the laser shock peening surface 54. The amplitude and quantity of these shockwave determine the depth and intensity of compressive stresses.

Referring to FIGS. 5 and 6, during the bare laser shock peening process described above a recast layer 11 of melted and resolidified material of the metallic substrate 10 is formed directly above the compressive pre-stressed regions 56 which extend inward into the article from the bare laser shock peened surfaces 55. The recast layer 11 may cause article to exhibit a lower LSP effect as compared to an article laser shock peened using an ablative coating. The article may also fail, by circumventing the LSP deep compressive stress, if cracks or defects can initiate in the recast layer and propagate around the LSP area or compressive pre-stressed region. The recast layer 11 is typically brittle.

The recast layer 11 reduces HCF strength of the fan blade 8 which was laser shock peened along the leading edge of the airfoil 34. The same is true for compressor and turbine blade airfoils. To that end, as part of a method of making a laser shock peened article 12, just the recast layer 11 above the pre-stressed region 56 is removed using a vibratory process with an abrasive media. Several vibratory processes are suitable. One suitable vibratory process is an abrasive tumbling process with the article being packed within a bed of abrasive particles and shaken during a tumbling cycle for a period of time. The period of time of the tumbling cycle may be in a range of several minutes to several hours and the tumbling cycle may incorporate vibratory frequencies in a range from a few Hz to several hundred Hz. The abrasive media may include stones characterized as having a size range from about 0.2 inches to 1.0 inches.

Another suitable vibratory process is mechanical peening of the article with small particles which either abrade and/or shatter a brittle surface of the recast layer. In a more particular embodiment the small particles are glass beads. Another suitable vibratory process is abrasive polishing which includes loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article. Another suitable process is a combination of two or more of the above mentioned processes. All three of the above mentioned vibratory processes may be used and particularly in the following order: the abrasive tumbling, the mechanical peening, and the abrasive polishing.

Vibratory finishing processes well known in the art for removing burrs from castings include barrelling, tumbling, rotating, agitating, spinning, shaking or centrifugal processes, where one or more workpieces are placed in a container or similar device with an abrasive medial or abrading elements that displace portions of the workpiece during the vibratory finishing process. The vibratory finishing process can be performed with or without a solution in the container. Tumbling processes have been used for many years during the manufacture of a wide variety of articles for surface preparation or treatment. For example, tumbling has been used for abrading, polishing, rough cutting, deburring, edge radiusing, descaling, surface texture or property improvement, cleaning, and destressing, among others. Various types of tumbling systems used include barrel, vibratory, and centrifugal, alone or in combinations, with or without liquid. Certain components for gas turbine engines, for example blades, vanes and nozzles, are complex in shape and have precision requirements for surface finish, including edges. The tumbling process has been used for surface treatment or preparation including removal of burrs and for the rounding of sharp edges produced during manufacture, as well as to achieve required surface finish.

One embodiment of the method of making a laser shock peened article 12 includes a preliminary abrasive grinding process to remove some level of recast prior to the tumbling or other vibratory processes with an abrasive media and other subsequent surface finishing operations or processes. The purpose is to insure that all recast and only recast is removed and eliminate the need for final inspections such as blue etch anodizing. An abrasive wheel, a simple commercially available abrasive media such as Scotch-Brite “Bristle Disks”, “Flap Brush” or “Combi Wheel” products from 3M, may be used in an automated abrasive wheel grinding process to pre-clean the airfoils prior to the abrasive tumbling, the mechanical peening, and/or the abrasive polishing processes. The abrasive such as the Scotch-Brite (TM) material which can also be impregnated with more aggressive abrasives such as alumina and garnet. These wheels are typically over 1 inch in diameter and over one inch in height and attached to the shaft of a small motor, electric or pneumatically driven. A pair of motors/abrasive wheel assemblies may be placed in a vertical spinning orientation such that their outer diameters are a few mils apart when spinning.

After the laser shock peening process the airfoil is passed between the two spinning abrasive wheels, which are flexible, such that surface imperfections such as LSP induced recast is abrasively removed. These abrasive wheel finishing processes already are used at various stages in the manufacture of a compressor airfoil and the use for the method of making a laser shock peened article 12 with the bare laser shock peening process would simply involve moving one of these processes immediately after the LSP process.

While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims. 

1. A method of making a laser shock peened article comprising the following steps: (a) bare laser shock peening a bare metallic laser shock peening surface of a substrate of the article without using an ablative coating on the metallic surface and forming a pre-stressed region having deep compressive residual stresses imparted by laser shock peening extending into the article from a laser shock peened surface and a recast layer above the laser shock peened surface and the pre-stressed region, and (b) removing just the recast layer with an abrasive vibratory process with an abrasive media.
 2. A method as claimed in claim 1 further comprising the vibratory process including an abrasive tumbling process including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle for a period of time.
 3. A method as claimed in claim 2 further comprising the period of time of the tumbling cycle being in a range of several minutes to several hours.
 4. A method as claimed in claim 3 further comprising the tumbling cycle being run at vibratory frequencies in a range from a few Hz to several hundred Hz.
 5. A method as claimed in claim 2 wherein the abrasive media includes stones characterized as having a size range from about 0.2 inches to 1.0 inches.
 6. A method as claimed in claim 1 further comprising the vibratory process including mechanical peening of the article with small particles which either abrade and/or shatter a brittle surface of the recast layer.
 7. A method as claimed in claim 6 further wherein the small particles are glass beads.
 8. A method as claimed in claim 1 further comprising the vibratory process being an abrasive polishing process including loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article.
 9. A method as claimed in claim 1 further comprising the vibratory process including a combination of two or more of the following processes: an abrasive tumbling process including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle for a period of time, a mechanical peening process including peening the article with small particles which either abrade and/or shatter a brittle surface of the recast layer, and an abrasive polishing process including loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article.
 10. A method as claimed in claim 1 further comprising the bare laser shock peening including providing a confining medium over the bare metallic surface and firing a laser beam through the confining medium onto the bare surface with sufficient power to vaporize material of the substrate to form the pre-stressed region having deep compressive residual stresses imparted by laser shock peening extending into the article and the recast layer above the pre-stressed region.
 11. A method as claimed in claim 1 wherein the article is a gas turbine engine airfoil having a leading edge and pressure and suction sides and the bare laser shock peening is performed at least on a portion of the bare metallic surface along the leading edge on both the pressure and suction sides of the airfoil simultaneously.
 12. A method as claimed in claim 11 further comprising the vibratory process being an abrasive tumbling process including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle for a period of time.
 13. A method as claimed in claim 12 further comprising the period of time of the tumbling cycle being in a range of several minutes to several hours.
 14. A method as claimed in claim 13 further comprising the tumbling cycle being run at vibratory frequencies in a range from a few Hz to several hundred Hz.
 15. A method as claimed in claim 12 wherein the abrasive media includes stones characterized as having a size range from about 0.2 inches to 1.0 inches.
 16. A method as claimed in claim 11 further comprising the vibratory process being a mechanical peening including peening the article with small particles which either abrade and/or shatter a brittle surface of the recast layer.
 17. A method as claimed in claim 16 further wherein the small particles are glass beads.
 18. A method as claimed in claim 11 further comprising the vibratory process being an abrasive polishing process including loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article.
 19. A method as claimed in claim 11 further comprising the bare laser shock peening including providing a confining medium over the bare metallic surface and firing a laser beam through the confining medium onto the bare surface with sufficient power to vaporize material of the substrate to form the pre-stressed region having deep compressive residual stresses imparted by laser shock peening extending into the article and the recast layer above the pre-stressed region.
 20. A method as claimed in claim 1 further comprising a preliminary abrasive grinding process to pre-clean the laser shock peened surface of the article after the bare laser shock peening and prior to the removing just the recast layer with an abrasive vibratory process.
 21. A method as claimed in claim 20 wherein the preliminary abrasive grinding process includes using a flexible abrasive wheel to pre-clean the laser shock peened surface.
 22. A method as claimed in claim 11 further comprising: a preliminary abrasive grinding process to pre-clean the laser shock peened surface of the airfoil after the bare laser shock peening and prior to the removing just the recast layer with an abrasive vibratory process, the preliminary abrasive grinding process including running the laser shock peened surface along the leading edge of the airfoil between two slightly spaced apart and spinning flexible abrasives wheels to pre-clean the laser shock peened surface on both the pressure and suction sides of the airfoil.
 23. A laser shock peened article comprising: a laser shock peened surface of a substrate of the article, a pre-stressed region having deep compressive residual stresses imparted by laser shock peening extending into the article from the laser shock peened surface, the laser shock peened surface and the pre-stressed region having been formed using bare laser shock peening, the bare laser shock peening a bare metallic surface of a substrate of the article without using an ablative coating on the metallic surface and forming a pre-stressed region having deep compressive residual stresses imparted by laser shock peening extending into the article and a recast layer above the pre-stressed region, and removing just the recast layer with an abrasive vibratory process with an abrasive media.
 24. An article as claimed in claim 23 wherein the vibratory process includes one or more of the following processes: an abrasive tumbling process including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle for a period of time, a mechanical peening process including peening the article is with small particles which either abrade and/or shatter a brittle surface of the recast layer, and an abrasive polishing process including loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article.
 25. An article as claimed in claim 23 further comprising the laser shock peened surface being located along a leading edge of a gas turbine engine airfoil.
 26. An article as claimed in claim 25 wherein the vibratory process includes one or more of the following processes: an abrasive tumbling process including packing the article within a bed of abrasive particles and shaking the article within the bed during a tumbling cycle for a period of time, a mechanical peening process including peening the article with small particles which either abrade and/or shatter a brittle surface of the recast layer, and an abrasive polishing process including loading the article into a bed of paste including abrasive particles and oscillating the bed across and around the article. 